The present invention relates to radioactivity measuring apparatus and methods. More particularly, the invention relates to methods and apparatus for accurately and reliably measuring airborne uranium and transuranium elements, particularly in the presence of naturally occurring radon and thoron radioactivity.
When handling or working with uranium or transuranic elements (a transuranic element is an element having an atomic number higher than that of uranium, such as plutonium), it is a common occurrence for small uranium or transuranic particles to become airborne. As even small particles of these elements are highly radioactive, it is important to monitor the air in the vicinity of such elements for the presence of uranium or transuranic particles. A high concentration of such particles in the air poses a health hazard if such air is breathed by workers or exhausted to the environment.
Uranium and transuranium elements emit alpha radiation (hereafter called "artificial radioactivity"). Other commonly occurring non-transuranic elements, such as radon and thoron, also emit alpha radiation (hereafter called "natural radioactivity"). Unfortunately, the maximum permissible concentrations of artificial radioactivity in air are very low compared to the concentrations of natural radioactivity already in the air. Thus, in order to measure accurately low concentrations of alpha particles attributable to artificial radioactivity, there is a need to subtract the number of alpha particles attributable to natural radioactivity from the total number of alpha particles present in a sample of air.
Various methods are known in the art for subtracting natural alpha radioactivity from the measurement of artificial radioactivity. One method relies on the fact that the half life of natural radioactivity is shorter than the half life of artificial radioactivity. Apparatus employing such method draws air through a one or two inch diameter filter. The filter medium is in the form of a long strip on a reel. The filter is advanced continuously or periodically so that there is a delay between the time the sample is collected to the time the alpha radiation emitted from the sample is counted. During the delay period, the natural radioactivity decays to a greater extent than does the artificial radioactivity. Hence, when the alpha count is made (after the delay period), a major portion of the alpha count is attributable to just the artificial radioactivity. Disadvantageously, the airborne concentrations thus measured are not made in real time. Hence, if there is an inadvertent release of contaminated air (air containing high levels of artificial radioactivity) to the atmosphere, it is not detected until after the delay period, which may be up to an hour or more. Further, in order to measure very low concentrations of artificial radioactivity, the delay between collecting and counting must be several hours in order to allow the natural radioactivity to decay completely out of the measurement. Such a long delay is unacceptable or impractical in many situations.
Another method known in the art for subtracting natural alpha radioactivity involves an inertial separation. This method assumes that most airborne particles emitting natural radioactivity are lodged on very small particles, while airborne particles emitting artificial radioactivity are lodged on larger particles. An impactor device is therefore used to separate large airborne particles from small airborne particles. The impactor device directs an air stream toward a surface. The air stream is made to change direction by ninety degrees. Larger airborne particles are not able to traverse the ninety degree bend in the air stream and are impinged on a surface at the bend. Smaller particles follow the air stream around the and are exhausted from the impactor device. The surface on which the larger particles are impacted includes the face of an alpha radioactivity detector which has been smeared with a sticky material. The resulting alpha count obtained from the detector is thus based primarily on the larger particles, which is presumed to be the artificial radioactivity. Unfortunately, this presumption is not always true. The particle sizes of artificial radioactivity vary depending upon the way the particles are formed. For mechanically generated particles (e.g., milling, drilling, grinding, etc.), the particle sizes tend to be large. For particles generated by heat or chemical reaction, however, the particle sizes may be much smaller, and in many cases may not be impinged on the detector. Further, in dusty environments, the buildup of dust on the detector absorbs much of the alpha radioactivity, thus lowering the efficiency of the detector. Moreover, the dust buildup on the detector is not uniform (as would be the case for air drawn through a filter), but is impacted over a very small area under each impactor nozzle. Hence, the accuracy of the measurement, particularly over time, is compromised.
Still another method known in the art to subtract natural alpha radioactivity from the measurement of artificial radioactivity involves the simultaneous measurement of alpha and beta radioactivity. This method is premised on the theory that natural radioactivity always includes both an alpha particle and a beta particle, whereas artificial radioactivity includes just an alpha particle. In accordance with this method, a thin window proportional counter is used to count both alpha and beta radioactivity from a fixed filter. Simply stated, if a beta particle is measured in quasi coincidence with an alpha particle, it means that the alpha particle is from natural radioactivity and is subtracted from the gross count. If an alpha particle is counted alone, it means that it was emitted by artificial radioactivity.
There are also several subtraction methods known in the art involving alpha spectroscopy. In accordance with such methods, an alpha spectrum is measured using conventional alpha spectroscopy techniques. Such an alpha spectrum is shown in FIG. 1, and shows the number of alpha particles detected (identified as "relative counts" along the vertical axis) versus the alpha particle energy (shown in units of millions of electron volts, or MeV, along the horizontal axis). The alpha spectrum shown in FIG. 1 is from a filter which has collected dust from the air for 65 hours. In FIG. 1, the 7.68 MeV alpha energy is from a daughter of radon (natural radioactivity). The alpha energy 8.78 MeV is from a daughter of thoron (also natural radioactivity). However, for the reasons set forth below, both thoron and radon daughter products contribute to the 6.0 MeV and lower alpha energies. Hence, if one desires to measure the radioactivity of plutonium-239 (having an energy of 5.2 MeV, and considered as artificial radioactivity), it is not possible to simply measure the 5.2 MeV alpha energy and assume it is all attributable to plutonium-239.
Alpha particles are not very penetrating. A plain piece of paper stops them. Thus, alpha particles cannot travel more than a few inches in air before giving up all energy to air molecules. This property of alpha particles complicates spectroscopy because alpha particles lose energy getting through the dust layer on the filter and the 1/8 to 1/4 inch of air between the filter and a detector. This scattering of alpha particles results in an energy-spectrum having broad peaks with tails on the lower energy side of the peak. The tail represents alpha particles which have lost more energy than those in the peak. The tail goes all the way to zero energy.
The energies of alpha particles from the artificial radioactivity of interest are all lower than the energies from natural radioactivity. FIG. 1 shows the position of the peak for plutonium-239 (approximately 5.2 MeV) and uranium-238 (approximately 4 MeV) if they were present in the sample.
Thus, the main problem in alpha spectroscopy is to determine the counting rate of alpha particles from natural radioactivity which have scattered down into the lower energy areas (frequently termed "background" radiation) so that it can be subtracted to determine the net counting rate from artificial radioactivity.
Several techniques are known in the spectroscopic art for subtracting the natural radioactivity scattered into the plutonium or uranium regions of interest. Because the present invention also relies on alpha spectroscopy to measure artificial radioactivity, a brief description of these prior techniques will be presented.
A first alpha spectroscopy method is known as the two region method. In accordance with the tworegion method, the alpha spectrum is divided into two regions, as shown in FIG. 2. One region includes all the counts above the plutonium energy (5.2 MeV). The other region includes all the counts, including the plutonium energy region, below 5.2 MeV. In FIG. 2, these two regions are labelled G.sub.B and G.sub.b. Hereafter G is the symbol for gross counts. By sampling in an area without artificial radioactivity, a ratio (R) of G.sub.b /G.sub.B is determined for natural radioactivity, such that: EQU G.sub.b =RG.sub.B ( 1)
If the sampler is placed in an area where plutonium may be present, G.sub.B is measured and multiplied by R (previously determined) and the product is subtracted from the total activity measured in the artificial radioactivity region of interest (G.sub.O). If the result (N.sub.O) is significantly greater than zero, artificial radioactivity is assumed to be present. In mathematical terms: EQU N.sub.O =G.sub.O -G.sub.b ( 2)
where G.sub.O is the total radioactivity in the region of interest and N.sub.O is the net radioactivity in the region of interest.
Because the ratio R is not constant, a modified two-region method has also been used. In the modified two-region method the energy regions G.sub.B and G.sub.b are reduced to only those counts in the 6 MeV peak and those in the plutonium or uranium peaks respectively, as shown in FIG. 3. The procedure for computing the net count due to artificial radioactivity is the same as in Eqs. (1) and (2).
The modified two-region method produces a value of R that is less variable than that produced using the two-region method. However, the ratio R is generally still too variable to detect small differences in concentration in the region of interest.
To improve upon the two-region and modified two-region background subtraction techniques, a method was introduced at Los Alamos National Laboratory known as the four region method. The four-region method divides the energy spectrum into four regions, as shown in FIG. 4. In accordance with the four-region method, it is presumed that the gross alpha particle counts in each of the four regions are related by: EQU G.sub.1 /G.sub.2 =K G.sub.3 /G.sub.4 ( 3)
where K is a proportionality constant determined empirically by sampling air where only natural radioactivity is present; G.sub.1 is the gross alpha count in the first region for natural radioactivity; G.sub.2 is the gross alpha count in the second region; G.sub.3 is the gross alpha count in the third region; and G.sub.4 is the gross alpha count in the fourth region.
When sampling air from an area where artificial radioactivity is present, the following equation is used by the four-region method to compute the net count due to artificial radioactivity: EQU N.sub.O =G.sub.1 -K * G.sub.2 * G.sub.3 /G.sub.4 ( 4)
In Eq. (4), it is noted that the background radiation, G.sub.b, is K * G.sub.2 * G.sub.3 /G.sub.4.
The four-region method provides a marked improvement over the two region methods previously described. However, even the four region method suffers from the fact that K is not constant for all conditions of natural radioactivity. A three-region method has thus been proposed as an alternative. FIG. 5 shows the regions used in the three-region method. Mathematically the counts due to plutonium are expressed as follows: EQU N.sub.O =G.sub.O -[K.sub.1 G.sub.1 +K.sub.2 G.sub.2 ] (5)
where K.sub.1 and K.sub.2 are empirically determined by sampling air where only natural radioactivity is present.
Basically, the three-region method uses the counts in regions G.sub.1 and G.sub.2 to interpolate linearly the background radiation, G.sub.b, between them, i.e., K.sub.1 G.sub.1 +K.sub.2 G.sub.2 =G.sub.b.
Disadvantageously, all of the above-described alpha spectroscopy methods suffer from one or more drawbacks.
First, the methods assume that the shape of the energy spectrum of each alpha particle is constant. In fact, the resolution of the alpha particle energies changes constantly. When a filter is new, the resolution tends to be poor, i.e., the alpha energy peaks are broad and tend to scatter more into the lower regions. This is most likely due to initial penetration of the filter paper by small dust particles carrying the naturally radioactive atoms. The alpha particles emitted by the radioactive atoms lose energy by interacting with filter material. As sampling goes on, the resolution tends to improve. The most likely explanation for the improved resolution is that the collected dust prohibits penetration of other dust particles into the filter. The result is less loss of energy getting to the detector and better resolution. FIGS. 1, 2 and 3 show the spectrum of a filter which had been used to collect dust for 65 hours. In contrast, FIGS. 4 and 5 show the spectrum of a filter which has been used to collect dust for only 12 hours. The 12 hour filter provides a much poorer resolution because the scattering of the alpha particles to the lower energies is much greater than with the 65 hour filter.
In addition to this rather slow change from poorer to better resolution, there also appears to be random changes over shorter time periods. This is believed to be due to the position of the radioactive atom in relation to the dust particle to which it is attached. If an alpha particle has to travel through its carrier dust particle to get to the detector, for example, it will lose more energy than if it is on the side of the particle facing the detector. Particle size distributions also change with time. This would also change the shape of the alpha spectrum. The bigger the dust particle, the poorer the resolution.
Another problem associated with alpha spectroscopy is illustrated in FIG. 6. FIG. 6 shows the alpha spectrum of a filter which had only been used to collect dust for nine hours. The resolution is much better than with the 12 hour collection. In fact, the resolution of the 7.68 MeV peak is almost as good as the 65 hour filter. However, the resolution of the 6 MeV peak is poorer. This illustrates another complicating factor. The resolution of the three naturally occurring alpha peaks is not necessarily the same, nor does there appear to be a constant relationship among them. They appear to act independently.
The first three methods (two-region, modified two-region, and four-region) assume that the relative amounts of the three alpha energies of natural radioactivity stay constant with time. In fact, because of their different half lives, the relative proportions of the peaks change with time. At the beginning of a sampling period, a first 6 MeV peak with a 3 minute half life predominates. Within a few hours, the 7.68 MeV peak predominates because it has a 28 minute half life. For filters left to collect dust for days, the 8.78 MeV peak and a second 6.0 MeV peak begin to dominate the spectrum because their half life is approximately 10 hours. In addition to these trends, short term variations also take place depending on local conditions such as ventilation exhaust rates, atmospheric conditions, etc.
The last method (three region) assumes linearity. In fact, however, the leading edge of the energy spectrum is exponential.
In view of the above, it is evident that what is needed is an improved alpha spectroscopy method of subtracting natural alpha radioactivity from the measurement of artificial radioactivity that does not assume a constant shape of the energy spectrum, thereby providing good resolution independent of time, i.e., providing just as good a resolution at 20 minutes as is obtained at one hour or at 65 hours. Further, it is evident that a method is needed that does not incorrectly assume that the relative amounts of the alpha energies stay constant with time or that the leading edge of the energy spectrum is linear. The present invention advantageously addresses these and other needs.